1. Field of the Invention
The present invention relates to an image reader having an optical means for reading an image of a document at an arbitrary magnification.
2. Description of Related Art
In the image reader as mentioned above, light reflected from a document to be read is focused on an image sensor by a zooming lens.
In such an image reader, the distance (a) defined between a reading position of a document and the zooming lens and the distance (b) defined between the zooming lens and the image sensor are varied so as to maintain the following relation defined by two equations as follows; EQU a=f (1--1/L..beta.) EQU b=f (1--.beta./L)
wherein
f: focal distance of the zooming lens;
L: magnification for reading;
.beta.: reduction ratio of the zooming lens upon reading a document.
In order to vary the magnification in the image reader of this type, the zooming lens and the image sensor are positioned at respective positions corresponding to the equal reading magnification with the use of an adjusting mechanism for the magnification and the focusing state. This adjusting function is performed by reading patterns provided for detecting the magnification and the focusing state which are formed on the undersurface of a document scale arranged near the top edge of a document. Namely, the magnification is calculated from a distance x defined between two black patterns and the focusing state is detected from output data obtained by reading a black and white stripe pattern formed between two black patterns.
Thereafter, the zooming lens and the image sensor are positioned at respective positions corresponding to a desired magnification by varying the distances (a) and (b) according to the equations mentioned above.
It is also possible to adjust the magnification starting from respective positions which are calculated corresponding to each given magnification. However, this method is not so actual since it is time consuming because of complicated processing for calculating moving amounts of the zooming lens and the image sensor. In order to vary the magnification at a high accuracy in the image reader of this type, possible aberration from the nominal focal distance of the zooming lens and possible error upon mounting it on a mounting means therefor, especially in the vertical direction, cannot be neglected. Ordinally, the zooming lens has an aberration of order of .+-.0.5% of the nominal focal distance.
In order to compensate these aberrations, a correction factor .alpha. is introduced upon calculating the distances a and b as follows; EQU a=.alpha..f (1--1/L.beta.) EQU b=.alpha..f (1--.beta./L).
This correction factor .alpha. is estimated from the real aberration of the focal distance measured from individual zooming lens after mounting it.
However, this method is time-consuming in measuring the aberration of the focal distance f and in estimating the correction factor .alpha. and, therefore, the production cost becomes high.